Continuous data replication

ABSTRACT

In a first embodiment, a method and computer program product for use in a storage system comprising quiescing IO commands the sites of an ACTIVE/ACTIVE storage system, the active/active storage system having at least two storage sites communicatively coupled via a virtualization layer, creating a change set, unquiescing IO commands by the virtualization layers, transferring data of a change set to the other sites of the active/active storage system by the virtualization layer, and flushing the data by the virtualization layer. 
     In a second embodiment, a method and computer program product for use in a storage system comprising fracturing a cluster of an active/active storage system; wherein the cluster includes at least two sites, stopping IO on a first site of the cluster; and rolling to a point in time on the first site.

RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.13/339,780 filed on Dec. 29, 2011 now U.S. Pat. No. 9,032,160, thecontent and teachings of which are hereby incorporated by reference intheir entirety.

A portion of the disclosure of this patent document may contain commandformats and other computer language listings, all of which are subjectto copyright protection. The copyright owner has no objection to thefacsimile reproduction by anyone of the patent document or the patentdisclosure, as it appears in the Patent and Trademark Office patent fileor records, but otherwise reserves all copyright rights whatsoever.

TECHNICAL FIELD

This invention relates to data replication.

BACKGROUND

Computer data is vital to today's organizations, and a significant partof protection against disasters is focused on data protection. Assolid-state memory has advanced to the point where cost of memory hasbecome a relatively insignificant factor, organizations can afford tooperate with systems that store and process terabytes of data.

Conventional data protection systems include tape backup drives, forstoring organizational production site data on a periodic basis. Suchsystems suffer from several drawbacks. First, they require a systemshutdown during backup, since the data being backed up cannot be usedduring the backup operation. Second, they limit the points in time towhich the production site can recover. For example, if data is backed upon a daily basis, there may be several hours of lost data in the eventof a disaster. Third, the data recovery process itself takes a longtime.

Another conventional data protection system uses data replication, bycreating a copy of the organization's production site data on asecondary backup storage system, and updating the backup with changes.The backup storage system may be situated in the same physical locationas the production storage system, or in a physically remote location.Data replication systems generally operate either at the applicationlevel, at the file system level, or at the data block level.

Current data protection systems try to provide continuous dataprotection, which enable the organization to roll back to any specifiedpoint in time within a recent history. Continuous data protectionsystems aim to satisfy two conflicting objectives, as best as possible;namely, (i) minimize the down time, in which the organization productionsite data is unavailable, during a recovery, and (ii) enable recovery asclose as possible to any specified point in time within a recenthistory.

Continuous data protection typically uses a technology referred to as“journaling,” whereby a log is kept of changes made to the backupstorage. During a recovery, the journal entries serve as successive“undo” information, enabling rollback of the backup storage to previouspoints in time. Journaling was first implemented in database systems,and was later extended to broader data protection.

One challenge to continuous data protection is the ability of a backupsite to keep pace with the data transactions of a production site,without slowing down the production site. The overhead of journalinginherently requires several data transactions at the backup site foreach data transaction at the production site. As such, when datatransactions occur at a high rate at the production site, the backupsite may not be able to finish backing up one data transaction beforethe next production site data transaction occurs. If the production siteis not forced to slow down, then necessarily a backlog of un-logged datatransactions may build up at the backup site. Without being able tosatisfactorily adapt dynamically to changing data transaction rates, acontinuous data protection system chokes and eventually forces theproduction site to shut down.

SUMMARY

In a first embodiment, a method and computer program product for use ina storage system comprising quiescing IO commands the sites of anACTIVE/ACTIVE storage system, the active/active storage system having atleast two storage sites communicatively coupled via a virtualizationlayer, creating a change set, unquiescing IO commands by thevirtualization layers, transferring data of a change set to the othersites of the active/active storage system by the virtualization layer,and flushing the data by the virtualization layer.

In a second embodiment, a method and computer program product for use ina storage system comprising fracturing a cluster of an active/activestorage system; wherein the cluster includes at least two sites,stopping IO on a first site of the cluster; and rolling to a point intime on the first site.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features, and advantages of embodiments disclosed herein may bebetter understood by referring to the following description inconjunction with the accompanying drawings. The drawings are not meantto limit the scope of the claims included herewith. For clarity, notevery element may be labeled in every figure. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments, principles, and concepts. Thus, features and advantages ofthe present disclosure will become more apparent from the followingdetailed description of exemplary embodiments thereof taken inconjunction with the accompanying drawings in which:

FIG. 1 is a simplified illustration of a data protection system, inaccordance with an embodiment of the present invention;

FIG. 2 is a simplified illustration of a write transaction for ajournal, in accordance with an embodiment of the present invention;

FIG. 3 is a simplified illustration of sites with a virtual servicelayer, in accordance with an embodiment of the present invention;

FIG. 4 is a simplified illustration of sites with a virtual servicelayer with write caches, in accordance with an embodiment of the presentinvention;

FIG. 5 is a simplified example of an embodiment of a method for creatinga snapshot for the two sites, in accordance with an embodiment of thepresent disclosure;

FIG. 6 is an alternative simplified illustration of sites with writecaches illustrating Point in Time access, in accordance with anembodiment of the present invention;

FIG. 7 is a simplified example of an embodiment of a method foraccessing a Point in Time, in accordance with an embodiment of thepresent disclosure;

FIG. 8 is a simplified example of an embodiment of a method for synchingan image across two sites, in accordance with an embodiment of thepresent disclosure;

FIG. 9 is an example of an embodiment of an apparatus that may utilizethe techniques described herein, in accordance with an embodiment of thepresent invention; and

FIG. 10 is an example of an embodiment of a method embodied on acomputer readable storage medium that may utilize the techniquesdescribed herein, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

In some embodiments of the instant disclosure, journal based replicationmay be integrated with a virtual service layer. In certain embodiments,one or more splitters may be integrated into the virtual service layer.In further embodiments, the virtual service lay may span multiple sites,and the sites may be at different geographic locations. In certainembodiments, each site may have one or more nodes and each node may havea splitter. In most embodiments, multiple sites linked via a virtualservice layer may be referred to herein as an ACTIVE/ACTIVE storagesystem. In at least some embodiments, the virtual service layer mayenable multiple sites to present the appearance of the same volume orvirtual volume. In most embodiments, a host or virtual machine accessingany of the sites with the virtual volume may see the same data on thevirtual volume as any other site. In further embodiments, each site mayreserve a portion of the virtual volume for active access.

In some embodiments of the instant disclosure, access to point in timeimages may be enabled in a replicated environment. In some embodimentsof the instant disclosure, access to point in time images may be enabledin an active/active storage system, for each of the sites. In at leastsome environments, the replicated environments may have a virtualservice layer. In certain embodiments, an UNDO stream in a journal maybe used to roll a site to a point in time (PIT). In most embodiments,each time a change set is to be pushed to a volume, the undo of thechange set may be read and sent to a data protection appliance, and thechange set may be written to the journal. In further embodiments, it maybe enabled to access any change set as a point in time. In mostembodiments, it is access to any point in time is enabled without havinga second copy of the data.

The following definitions are employed throughout the specification andclaims.

BACKUP SITE—may be a facility where replicated production site data isstored; the backup site may be located in a remote site or at the samelocation as the production site;

CLONE—a clone may be a copy or clone of the image or images, drive ordrives of a first location at a second location;

DELTA MARKING STREAM—may mean the tracking of the delta between theproduction and replication site, which may contain the meta data ofchanged locations, the delta marking stream may be kept persistently onthe journal at the production site of the replication, based on thedelta marking data the DPA knows which locations are different betweenthe production and the replica and transfers them to the replica to makeboth sites identical.

DPA—may be Data Protection Appliance a computer or a cluster ofcomputers, or a set of processes that serve as a data protectionappliance, responsible for data protection services including inter aliadata replication of a storage system, and journaling of I/O requestsissued by a host computer to the storage system;

RPA—may be replication protection appliance, is another name for DPA.

HOST—may be at least one computer or networks of computers that runs atleast one data processing application that issues I/O requests to one ormore storage systems; a host is an initiator with a SAN;

HOST DEVICE—may be an internal interface in a host, to a logical storageunit;

IMAGE—may be a copy of a logical storage unit at a specific point intime;

INITIATOR—may be a node in a SAN that issues I/O requests;

JOURNAL—may be a record of write transactions issued to a storagesystem; used to maintain a duplicate storage system, and to rollback theduplicate storage system to a previous point in time;

LOGICAL UNIT—may be a logical entity provided by a storage system foraccessing data from the storage system;

LUN—may be a logical unit number for identifying a logical unit;

PHYSICAL STORAGE UNIT—may be a physical entity, such as a disk or anarray of disks, for storing data in storage locations that can beaccessed by address;

PRODUCTION SITE—may be a facility where one or more host computers rundata processing applications that write data to a storage system andread data from the storage system;

SAN—may be a storage area network of nodes that send and receive I/O andother requests, each node in the network being an initiator or a target,or both an initiator and a target;

SOURCE SIDE—may be a transmitter of data within a data replicationworkflow, during normal operation a production site is the source side;and during data recovery a backup site is the source side;

SNAPSHOT—a Snapshot may refer to differential representations of animage, i.e. the snapshot may have pointers to the original volume, andmay point to log volumes for changed locations. Snapshots may becombined into a snapshot array, which may represent different imagesover a time period.

STORAGE SYSTEM—may be a SAN entity that provides multiple logical unitsfor access by multiple SAN initiators

TARGET—may be a node in a SAN that replies to I/O requests;

TARGET SIDE—may be a receiver of data within a data replicationworkflow; during normal operation a back site is the target side, andduring data recovery a production site is the target side;

WAN—may be a wide area network that connects local networks and enablesthem to communicate with one another, such as the Internet.

SPLITTER/PROTECTION AGENT: may be an agent running either on aproduction host a switch or a storage array which can intercept IO andsplit them to a DPA and to the storage array, fail IO redirect IO or doany other manipulation to the IO.

VIRTUAL VOLUME: may be a volume which is exposed to host by avirtualization layer, the virtual volume may be spanned across more thanone site

DISTRIBUTED MIRROR: may be a mirror of a volume across distance, eithermetro or geo, which is accessible at all sites.

BLOCK VIRTUALIZATION: may be a layer, which takes backend storagevolumes and by slicing concatenation and striping create a new set ofvolumes, which serve as base volumes or devices in the virtualizationlayer

MARKING ON SPLITTER: may be a mode in a splitter where intercepted IOsare not split to an appliance and the storage, but changes (meta data)are tracked in a list and/or a bitmap and I/O is immediately sent todown the IO stack.

FAIL ALL MODE: may be a mode of a volume in the splitter where all writeand read IOs intercepted by the splitter are failed to the host, butother SCSI commands like read capacity are served.

GLOBAL FAIL ALL MODE: may be a mode of a volume in the virtual layerwhere all write and read IOs virtual layer are failed to the host, butother SCSI commands like read capacity are served.

LOGGED ACCESS: may be an access method provided by the appliance and thesplitter, in which the appliance rolls the volumes of the consistencygroup to the point in time the user requested and let the host accessthe volumes in a copy on first write base.

VIRTUAL ACCESS: may be an access method provided by the appliance andthe splitter, in which the appliance exposes a virtual volume from aspecific point in time to the host, the data for the virtual volume ispartially stored on the remote copy and partially stored on the journal.

CDP: Continuous Data Protection, may refer to a full replica of a volumeor a set of volumes along with a journal which allows any point in timeaccess, the CDP copy is at the same site, and maybe the same storagearray of the production site

CRR: Continuous Remote Replica may refer to a full replica of a volumeor a set of volumes along with a journal which allows any point in timeaccess at a site remote to the production volume and on a separatestorage array.

A description of journaling and some techniques associated withjournaling may be described in the patent titled METHODS AND APPARATUSFOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION and with U.S.Pat. No. 7,516,287, which is hereby incorporated by reference.

A discussion of image access may be found in U.S. patent applicationSer. No. 12/969,903 entitled “DYNAMIC LUN RESIZING IN A REPLICATIONENVIRONMENT” filed on Dec. 16, 2010 assigned to EMC Corp., which ishereby incorporated by reference.

A discussion of journal based replication may be integrated with avirtual service layer. may be found in U.S. patent application Ser. Nos.13/077,256, 13/077,262, and 13/077,266, entitled “CONSISTENT REPLICATIONIN A GEOGRAPHICALLY DISPERSE ACTIVE ENVIRONMENT,” “INVERSE STARREPLICATION,” and “NETWORKED BASED REPLICATION OF DISTRIBUTED VOLUMES,”respectively, filed on Dec. 16, 2010 assigned to EMC Corp., which ishereby incorporated by reference.

Description of Embodiments Using of a Five State Journaling Process

Reference is now made to FIG. 1, which is a simplified illustration of adata protection system 100, in accordance with an embodiment of thepresent invention. Shown in FIG. 1 are two sites; Site I, which is aproduction site, on the right, and Site II, which is a backup site, onthe left. Under normal operation the production site is the source sideof system 100, and the backup site is the target side of the system. Thebackup site is responsible for replicating production site data.Additionally, the backup site enables rollback of Site I data to anearlier pointing time, which may be used in the event of data corruptionof a disaster, or alternatively in order to view or to access data froman earlier point in time.

During normal operations, the direction of replicate data flow goes fromsource side to target side. It is possible, however, for a user toreverse the direction of replicate data flow, in which case Site Istarts to behave as a target backup site, and Site II starts to behaveas a source production site. Such change of replication direction isreferred to as a “failover”. A failover may be performed in the event ofa disaster at the production site, or for other reasons. In some dataarchitectures, Site I or Site II behaves as a production site for aportion of stored data, and behaves simultaneously as a backup site foranother portion of stored data. In some data architectures, a portion ofstored data is replicated to a backup site, and another portion is not.

The production site and the backup site may be remote from one another,or they may both be situated at a common site, local to one another.Local data protection has the advantage of minimizing data lag betweentarget and source, and remote data protection has the advantage is beingrobust in the event that a disaster occurs at the source side.

The source and target sides communicate via a wide area network (WAN)128, although other types of networks are also adaptable for use withthe present invention.

In accordance with an embodiment of the present invention, each side ofsystem 100 includes three major components coupled via a storage areanetwork (SAN); namely, (i) a storage system, (ii) a host computer, and(iii) a data protection appliance (DPA). Specifically with reference toFIG. 1, the source side SAN includes a source host computer 104, asource storage system 108, and a source DPA 112. Similarly, the targetside SAN includes a target host computer 116, a target storage system120, and a target DPA 124.

Generally, a SAN includes one or more devices, referred to as “nodes”. Anode in a SAN may be an “initiator” or a “target”, or both. An initiatornode is a device that is able to initiate requests to one or more otherdevices; and a target node is a device that is able to reply torequests, such as SCSI commands, sent by an initiator node. A SAN mayalso include network switches, such as fiber channel switches. Thecommunication links between each host computer and its correspondingstorage system may be any appropriate medium suitable for data transfer,such as fiber communication channel links.

In an embodiment of the present invention, the host communicates withits corresponding storage system using small computer system interface(SCSI) commands.

System 100 includes source storage system 108 and target storage system120. Each storage system includes physical storage units for storingdata, such as disks or arrays of disks. Typically, storage systems 108and 120 are target nodes. In order to enable initiators to send requeststo storage system 108, storage system 108 exposes one or more logicalunits (LU) to which commands are issued. Thus, storage systems 108 and120 are SAN entities that provide multiple logical units for access bymultiple SAN initiators. Logical units are a logical entity provided bya storage system, for accessing data stored in the storage system. Alogical unit is identified by a unique logical unit number (LUN). In anembodiment of the present invention, storage system 108 exposes alogical unit 136, designated as LU A, and storage system 120 exposes alogical unit 156, designated as LU B.

In an embodiment of the present invention, LU B is used for replicatingLU A. As such, LU B is generated as a copy of LU A. In one embodiment,LU B is configured so that its size is identical to the size of LU A.Thus for LU A, storage system 120 serves as a backup for source sidestorage system 108. Alternatively, as mentioned hereinabove, somelogical units of storage system 120 may be used to back up logical unitsof storage system 108, and other logical units of storage system 120 maybe used for other purposes. Moreover, in certain embodiments of thepresent invention, there is symmetric replication whereby some logicalunits of storage system 108 are used for replicating logical units ofstorage system 120, and other logical units of storage system 120 areused for replicating other logical units of storage system 108.

System 100 includes a source side host computer 104 and a target sidehost computer 116. A host computer may be one computer, or a pluralityof computers, or a network of distributed computers, each computer mayinclude inter alia a conventional CPU, volatile and non-volatile memory,a data bus, an I/O interface, a display interface and a networkinterface. Generally a host computer runs at least one data processingapplication, such as a database application and an e-mail server.

Generally, an operating system of a host computer creates a host devicefor each logical unit exposed by a storage system in the host computerSAN. A host device is a logical entity in a host computer, through whicha host computer may access a logical unit. In an embodiment of thepresent invention, host device 104 identifies LU A and generates acorresponding host device 140, designated as Device A, through which itcan access LU A. Similarly, host computer 116 identifies LU B andgenerates a corresponding device 160, designated as Device B.

In an embodiment of the present invention, in the course of continuousoperation, host computer 104 is a SAN initiator that issues I/O requests(write/read operations) through host device 140 to LU A using, forexample, SCSI commands. Such requests are generally transmitted to LU Awith an address that includes a specific device identifier, an offsetwithin the device, and a data size. Offsets are generally aligned to 512byte blocks. The average size of a write operation issued by hostcomputer 104 may be, for example, 10 kilobytes (KB); i.e., 20 blocks.For an I/O rate of 50 megabytes (MB) per second, this corresponds toapproximately 5,000 write transactions per second.

System 100 includes two data protection appliances, a source side DPA112 and a target side DPA 124. A DPA performs various data protectionservices, such as data replication of a storage system, and journalingof I/O requests issued by a host computer to source side storage systemdata. As explained in detail hereinbelow, when acting as a target sideDPA, a DPA may also enable rollback of data to an earlier point in time,and processing of rolled back data at the target site. Each DPA 112 and124 is a computer that includes inter alia one or more conventional CPUsand internal memory.

For additional safety precaution, each DPA is a cluster of suchcomputers. Use of a cluster ensures that if a DPA computer is down, thenthe DPA functionality switches over to another computer. The DPAcomputers within a DPA cluster communicate with one another using atleast one communication link suitable for data transfer via fiberchannel or IP based protocols, or such other transfer protocol. Onecomputer from the DPA cluster serves as the DPA leader. The DPA clusterleader coordinates between the computers in the cluster, and may alsoperform other tasks that require coordination between the computers,such as load balancing.

In the architecture illustrated in FIG. 1, DPA 112 and DPA 124 arestandalone devices integrated within a SAN. Alternatively, each of DPA112 and DPA 124 may be integrated into storage system 108 and storagesystem 120, respectively, or integrated into host computer 104 and hostcomputer 116, respectively. Both DPAs communicate with their respectivehost computers through communication lines such as fiber channels using,for example, SCSI commands.

In accordance with an embodiment of the present invention, DPAs 112 and124 are configured to act as initiators in the SAN; i.e., they can issueI/O requests using, for example, SCSI commands, to access logical unitson their respective storage systems. DPA 112 and DPA 124 are alsoconfigured with the necessary functionality to act as targets; i.e., toreply to I/O requests, such as SCSI commands, issued by other initiatorsin the SAN, including inter alia their respective host computers 104 and116. Being target nodes, DPA 112 and DPA 124 may dynamically expose orremove one or more logical units.

As described hereinabove, Site I and Site II may each behavesimultaneously as a production site and a backup site for differentlogical units. As such, DPA 112 and DPA 124 may each behave as a sourceDPA for some logical units, and as a target DPA for other logical units,at the same time.

In accordance with an embodiment of the present invention, host computer104 and host computer 116 include protection agents 144 and 164,respectively. Protection agents 144 and 164 intercept SCSI commandsissued by their respective host computers, via host devices to logicalunits that are accessible to the host computers. In accordance with anembodiment of the present invention, a data protection agent may act onan intercepted SCSI commands issued to a logical unit, in one of thefollowing ways:

Send the SCSI commands to its intended logical unit.

Redirect the SCSI command to another logical unit.

Split the SCSI command by sending it first to the respective DPA. Afterthe DPA returns an acknowledgement, send the SCSI command to itsintended logical unit.

Fail a SCSI command by returning an error return code.

Delay a SCSI command by not returning an acknowledgement to therespective host computer.

A protection agent may handle different SCSI commands, differently,according to the type of the command. For example, a SCSI commandinquiring about the size of a certain logical unit may be sent directlyto that logical unit, while a SCSI write command may be split and sentfirst to a DPA associated with the agent. A protection agent may alsochange its behavior for handling SCSI commands, for example as a resultof an instruction received from the DPA.

Specifically, the behavior of a protection agent for a certain hostdevice generally corresponds to the behavior of its associated DPA withrespect to the logical unit of the host device. When a DPA behaves as asource site DPA for a certain logical unit, then during normal course ofoperation, the associated protection agent splits I/O requests issued bya host computer to the host device corresponding to that logical unit.Similarly, when a DPA behaves as a target device for a certain logicalunit, then during normal course of operation, the associated protectionagent fails I/O requests issued by host computer to the host devicecorresponding to that logical unit.

Communication between protection agents and their respective DPAs mayuse any protocol suitable for data transfer within a SAN, such as fiberchannel, or SCSI over fiber channel. The communication may be direct, orvia a logical unit exposed by the DPA. In an embodiment of the presentinvention, protection agents communicate with their respective DPAs bysending SCSI commands over fiber channel.

In an embodiment of the present invention, protection agents 144 and 164are drivers located in their respective host computers 104 and 116.Alternatively, a protection agent may also be located in a fiber channelswitch, or in any other device situated in a data path between a hostcomputer and a storage system.

What follows is a detailed description of system behavior under normalproduction mode, and under recovery mode.

In accordance with an embodiment of the present invention, in productionmode DPA 112 acts as a source site DPA for LU A. Thus, protection agent144 is configured to act as a source side protection agent; i.e., as asplitter for host device A. Specifically, protection agent 144replicates SCSI I/O requests. A replicated SCSI I/O request is sent toDPA 112. After receiving an acknowledgement from DPA 124, protectionagent 144 then sends the SCSI I/O request to LU A. Only after receivinga second acknowledgement from storage system 108 may host computer 104initiate another I/O request.

When DPA 112 receives a replicated SCSI write request from dataprotection agent 144, DPA 112 transmits certain I/O informationcharacterizing the write request, packaged as a “write transaction”,over WAN 128 to DPA 124 on the target side, for journaling and forincorporation within target storage system 120.

DPA 112 may send its write transactions to DPA 124 using a variety ofmodes of transmission, including inter alia (i) a synchronous mode, (ii)an asynchronous mode, and (iii) a snapshot mode. In synchronous mode,DPA 112 sends each write transaction to DPA 124, receives back anacknowledgement from DPA 124, and in turns sends an acknowledgement backto protection agent 144. Protection agent 144 waits until receipt ofsuch acknowledgement before sending the SCSI write request to LU A.

In asynchronous mode, DPA 112 sends an acknowledgement to protectionagent 144 upon receipt of each I/O request, before receiving anacknowledgement back from DPA 124.

In snapshot mode, DPA 112 receives several I/O requests and combinesthem into an aggregate “snapshot” of all write activity performed in themultiple I/O requests, and sends the snapshot to DPA 124, for journalingand for incorporation in target storage system 120. In snapshot mode DPA112 also sends an acknowledgement to protection agent 144 upon receiptof each I/O request, before receiving an acknowledgement back from DPA124.

For the sake of clarity, the ensuing discussion assumes that informationis transmitted at write-by-write granularity.

While in production mode, DPA 124 receives replicated data of LU A fromDPA 112, and performs journaling and writing to storage system 120. Whenapplying write operations to storage system 120, DPA 124 acts as aninitiator, and sends SCSI commands to LU B.

During a recovery mode, DPA 124 undoes the write transactions in thejournal, so as to restore storage system 120 to the state it was at, atan earlier time.

As described hereinabove, in accordance with an embodiment of thepresent invention, LU B is used as a backup of LU A. As such, duringnormal production mode, while data written to LU A by host computer 104is replicated from LU A to LU B, host computer 116 should not be sendingI/O requests to LU B. To prevent such I/O requests from being sent,protection agent 164 acts as a target site protection agent for hostDevice B and fails I/O requests sent from host computer 116 to LU Bthrough host Device B.

In accordance with an embodiment of the present invention, targetstorage system 120 exposes a logical unit 176, referred to as a “journalLU”, for maintaining a history of write transactions made to LU B,referred to as a “journal”. Alternatively, journal LU 176 may be stripedover several logical units, or may reside within all of or a portion ofanother logical unit. DPA 124 includes a journal processor 180 formanaging the journal.

Journal processor 180 functions generally to manage the journal entriesof LU B. Specifically, journal processor 180 (i) enters writetransactions received by DPA 124 from DPA 112 into the journal, bywriting them into the journal LU, (ii) applies the journal transactionsto LU B, and (iii) updates the journal entries in the journal LU withundo information and removes already-applied transactions from thejournal. As described below, with reference to FIGS. 2 and 3A-3D,journal entries include four streams, two of which are written whenwrite transaction are entered into the journal, and two of which arewritten when write transaction are applied and removed from the journal.

Reference is now made to FIG. 2, which is a simplified illustration of awrite transaction 200 for a journal, in accordance with an embodiment ofthe present invention. The journal may be used to provide an adaptor foraccess to storage 120 at the state it was in at any specified point intime. Since the journal contains the “undo” information necessary torollback storage system 120, data that was stored in specific memorylocations at the specified point in time may be obtained by undoingwrite transactions that occurred subsequent to such point in time.

Write transaction 200 generally includes the following fields:

one or more identifiers;

a time stamp, which is the date & time at which the transaction wasreceived by source side DPA 112;

a write size, which is the size of the data block;

a location in journal LU 176 where the data is entered;

a location in LU B where the data is to be written; and

the data itself.

Write transaction 200 is transmitted from source side DPA 112 to targetside DPA 124. As shown in FIG. 2, DPA 124 records the write transaction200 in four streams. A first stream, referred to as a DO stream,includes new data for writing in LU B. A second stream, referred to asan DO METADATA stream, includes metadata for the write transaction, suchas an identifier, a date & time, a write size, a beginning address in LUB for writing the new data in, and a pointer to the offset in the dostream where the corresponding data is located. Similarly, a thirdstream, referred to as an UNDO stream, includes old data that wasoverwritten in LU B; and a fourth stream, referred to as an UNDOMETADATA, include an identifier, a date & time, a write size, abeginning address in LU B where data was to be overwritten, and apointer to the offset in the undo stream where the corresponding olddata is located.

In practice each of the four streams holds a plurality of writetransaction data. As write transactions are received dynamically bytarget DPA 124, they are recorded at the end of the DO stream and theend of the DO METADATA stream, prior to committing the transaction.During transaction application, when the various write transactions areapplied to LU B, prior to writing the new DO data into addresses withinthe storage system, the older data currently located in such addressesis recorded into the UNDO stream.

By recording old data, a journal entry can be used to “undo” a writetransaction. To undo a transaction, old data is read from the UNDOstream in a reverse order, from the most recent data to the oldest data,for writing into addresses within LU B. Prior to writing the UNDO datainto these addresses, the newer data residing in such addresses isrecorded in the DO stream.

The journal LU is partitioned into segments with a pre-defined size,such as 1 MB segments, with each segment identified by a counter. Thecollection of such segments forms a segment pool for the four journalingstreams described hereinabove. Each such stream is structured as anordered list of segments, into which the stream data is written, andincludes two pointers—a beginning pointer that points to the firstsegment in the list and an end pointer that points to the last segmentin the list.

According to a write direction for each stream, write transaction datais appended to the stream either at the end, for a forward direction, orat the beginning, for a backward direction. As each write transaction isreceived by DPA 124, its size is checked to determine if it can fitwithin available segments. If not, then one or more segments are chosenfrom the segment pool and appended to the stream's ordered list ofsegments.

Thereafter the DO data is written into the DO stream, and the pointer tothe appropriate first or last segment is updated. Freeing of segments inthe ordered list is performed by simply changing the beginning or theend pointer. Freed segments are returned to the segment pool for re-use.

A journal may be made of any number of streams including less than ormore than 5 streams. Often, based on the speed of the journaling andwhether the backup is synchronous or a synchronous a fewer or greaternumber of streams may be used.

Image Access

Herein, some information is provided for conventional continuous dataprotection systems having journaling and a replication splitter whichmay be used in one or more embodiments is provided. A replication mayset refer to an association created between the source volume and thelocal and/or remote target volumes, and a consistency group contains oneor more replication sets. A snapshot may be the difference between oneconsistent image of stored data and the next. The exact time for closingthe snapshot may determined dynamically depending on replicationpolicies and the journal of the consistency group.

In synchronous replication, each write may be a snapshot. When thesnapshot is distributed to a replica, it may be stored in the journalvolume, so that is it possible to revert to previous images by using thestored snapshots. As noted above, a splitter mirrors may write from anapplication server to LUNs being protected by the data protectionappliance. When a write is requested from the application server it maybe split and sent to the appliance using a host splitter/driver(residing in the I/O stack, below any file system and volume manager,and just above any multipath driver (such as EMC POWERPATH), through anintelligent fabric switch, through array-based splitter, such as EMCCLARiiON.

There may be a number of image access modes. Image access may be used torestore production from the disaster recovery site, and to roll back toa previous state of the data. Image access may be also to temporarilyoperate systems from a replicated copy while maintenance work is carriedout on the production site and to fail over to the replica. When imageaccess is enabled, host applications at the copy site may be able toaccess the replica.

In virtual access, the system may create the image selected in aseparate virtual LUN within the data protection appliance. Whileperformance may be constrained by the appliance, access to thepoint-in-time image may be nearly instantaneous. The image may be usedin the same way as logged access (physical), noting that data changesare temporary and stored in the local journal. Generally, this type ofimage access is chosen because the user may not be sure which image, orpoint in time is needed. The user may access several images to conductforensics and determine which replica is required. Note that in knownsystems, one cannot recover the production site from a virtual imagesince the virtual image is temporary. Generally, when analysis on thevirtual image is completed, the choice is made to disable image access.

If it is determined the image should be maintained, then access may bechanged to logged access using ‘roll to image.’ When disable imageaccess is disabled, the virtual LUN and all writes to it may bediscarded.

In an embodiment of virtual access with roll image in background, thesystem first creates the image in a virtual volume managed by the dataprotection appliance to provide rapid access to the image, the same asin virtual access. Simultaneously in background, the system may roll tothe physical image. Once the system has completed this action, thevirtual volume may be discarded, and the physical volume may take itsplace. At this point, the system continues to function as if loggedimage access was initially selected. The switch from virtual to physicalmay be transparent to the servers and applications and the user may notsee any difference in access. Once this occurs, changes may be read fromthe physical volume instead of being performed by the appliance. Ifimage access is disabled, the writes to the volume while image accesswas enabled may be rolled back (undone). Then distribution to storagemay continue from the accessed image forward.

In some embodiments in physical logged access, the system rolls backward(or forward) to the selected snapshot (point in time). There may be adelay while the successive snapshots are applied to the replica image tocreate the selected image. The length of delay may depend on how far theselected snapshot is from the snapshot currently being distributed tostorage. Once the access is enabled, hosts may read data directly fromthe volume and writes may be handled through the DPA. The host may readthe undo data of the write and the appliance may store the undo data ina logged access journal. During logged access the distribution ofsnapshots from the journal to storage may be paused. When image accessis disabled, writes to the volume while image access was enabled(tracked in the logged access journal) may be rolled back (undone). Thendistribution to storage may continue from the accessed snapshot forward.

Disable image access may mean changes to the replica may be discarded orthrown away. It may not matter what type of access was initiated, thatis, logged or another type, or whether the image chosen was the latestor an image back in time. Disable image access effectively says the workdone at the disaster recovery site is no longer needed.

Delta Marking

A delta marker stream may contain the locations that may be differentbetween the latest I/O data which arrived to the remote side (thecurrent remote site) and the latest I/O data which arrived at the localside. In particular, the delta marking stream may include metadata ofthe differences between the source side and the target side. Forexample, every I/O reaching the data protection appliance for the source112 may be written to the delta marking stream and data is freed fromthe delta marking stream when the data safely arrives at both the sourcevolume of replication 108 and the remote journal 180 (e.g. DO stream).Specifically, during an initialization process no data may be freed fromthe delta marking stream; and only when the initialization process iscompleted and I/O data has arrived to both local storage and the remotejournal data, may be I/O data from the delta marking stream freed. Whenthe source and target are not synchronized, data may not be freed fromthe delta marking stream. The initialization process may start bymerging delta marking streams of the target and the source so that thedelta marking stream includes a list of all different locations betweenlocal and remote sites. For example, a delta marking stream at thetarget might have data too if a user has accessed an image at the targetsite.

The initialization process may create one virtual disk out of all theavailable user volumes. The virtual space may be divided into a selectednumber of portions depending upon the amount of data needed to besynchronized. A list of ‘dirty’ blocks may be read from the delta markerstream that is relevant to the area currently being synchronized toenable creation of a dirty location data structure. The system may beginsynchronizing units of data, where a unit of data is a constant amountof dirty data, e.g., a data that needs to be synchronized.

The dirty location data structure may provide a list of dirty locationuntil the amount of dirty location is equal to the unit size or untilthere is no data left. The system may begin a so-called ping pongprocess to synchronize the data. The process may transfer thedifferences between the production and replica site to the replica.

Virtual Service Layer

Typical server environments have one or more hosts access storage.Conventionally, some of the hosts may be virtual hosts or virtualmachines. Generally, each virtual machine or host has a LUN or logicalunit corresponding to storage space it may access. Typically, this LUNcorresponds to a portion of one or more physical disks mapped to the LUNor logical drive.

Conventional Server virtualization products may have developed thecapability to execute migrations of virtual machines, the underlyingstorage, or both to address load balancing and high availabilityrequirements with certain limitations. Typically, conventional solutionsusually require disruptive failover (i.e. failure of one site totransfer the processes to the backup site), merged SANs, and do not workwith heterogeneous products. Thus, in typical systems, if a VirtualMachine were migrated to another environment, such as a server atanother location outside of a site, the virtual machine would no longerhave read write access to the LUN. However, it is desirable to be ableto migrate a virtual machine and have it be able to have read writeaccess to the underlying storage.

In certain embodiments of the instant disclosure, storage resources areenabled to be aggregated and virtualized to provide a dynamic storageinfrastructure to complement the dynamic virtual server infrastructure.In an embodiment of the current invention, users are enabled to access asingle copy of data at different geographical locations concurrently,enabling a transparent migration of running virtual machines betweendata centers. In some embodiments, this capability may enable fortransparent load sharing between multiple sites while providing theflexibility of migrating workloads between sites in anticipation ofplanned events. In other embodiments, in case of an unplanned event thatcauses disruption of services at one of the data centers, the failedservices maybe restarted at the surviving site with minimal effort whileminimizing recovery time objective (RTO).

In some embodiments of the current techniques the IT infrastructureincluding servers, storage, and networks may be virtualized. In certainembodiments, resources may be presented as a uniform set of elements inthe virtual environment. In other embodiments of the current techniqueslocal and distributed federation is enabled which may allow transparentcooperation of physical data elements within a single site or twogeographically separated sites. In some embodiments, the federationcapabilities may enable collection of the heterogeneous data storagesolutions at a physical site and present the storage as a pool ofresources. In some embodiments, virtual storage is enabled to spanmultiple data centers

In some embodiments, virtual storage or a virtual storage layer may havea front end and a back end. The back end may consume storage volumes andcreate virtual volumes from the consumed volumes. The virtual volumesmay be made up of portions or concatenations of the consumed volumes.For example, the virtual volumes may stripped across the consumedvolumes or may be made up of consumed volumes running a flavor of RAID.Usually, the front-end exposes these volumes to hosts.

An example embodiment of a virtual service layer or virtual serviceappliance is EMC Corporation's Vplex®. In some embodiments of theinstant disclosure, a storage virtualization appliance has a backendexposes LUNs to hosts and a front-end which talks to storage arrays,which may enable data mobility. In certain embodiments, storage may beadded or removed from the virtual service layer transparently to theuser

In most embodiments, the virtual service layer enables cache coherency.Thus, in certain embodiments of the current techniques, the storagevolumes, in a virtualized server environment, which comprise theencapsulation of a virtual machine may be coherently co-located in twosites, enabling simultaneous, local access by the virtual machineregardless of whether the virtual machine is located on the local orremote site. In other embodiments, cooperative clustering of thevirtualization server nodes may allow for active/active, concurrentread/write access to one or more federated storage devices across thesites. In further embodiments, concurrent access may occur even if thedata has not yet been fully copied between the two sites. In at leastsome embodiments of the current techniques, it is enabled to referencethe source copy in this case, preserving seamless, continuous operation.

In certain embodiments of the current disclosure, movement of thevirtual machines between the two sites is facilitated. In someembodiments, LUN level access is active/active, any single virtualmachine may execute on only one node of the cluster. In furtherembodiments, enabling of migration of virtual machine instances mayenable the migration of the I/O load (specifically read workloads) tostorage devices located in the site where the active node resides forany given virtual machine.

In some embodiments of the current techniques, the ability to migrate aVM may be enabled through the use of one or more federated virtualvolume. In certain embodiments, a virtual machine or application maycommunicate through a network with a module which presents virtualvolumes to the application or virtual machine. In further embodimentsthe network may be a SAN. In at least some embodiments, this module mayprovide a level of abstraction between the storage and the requests forstorage made by a virtual machine or other application. In theseembodiments, the module may map the logical drive presented to the VM orapplication to the storage device. In certain embodiments, the modulemay be transparent to the storage request, the application or VMfunctioning as it is accessing a logical drive across a network. Inother embodiments the network may be a SAN. In other embodiments,regardless of location of the VM, the VM may attempt to reach the LUNprovided by the module, which may map the VM request to the appropriatestorage.

In some embodiments of the current invention, a clustering architectureenables servers at multiple data centers to have concurrent read andwrite access to shared block storage devices. In alternative embodimentsof the current invention, load sharing between multiple sites whileproviding the flexibility of migrating workloads between sites inanticipation of planned events such as hardware maintenance is enabled.In further embodiments, in case of an unplanned event that causesdisruption of services at one of the data centers, the failed servicesmay be quickly and easily restarted at the surviving site with minimaleffort.

In most embodiments, the module may communicate with a second module atthe second site to facilitate the one or more federated logical drive.In some embodiments, if a VM were to be moved from the first site to thesecond site the VM would attempt to access storage through the secondmodule. In most embodiments, the move would be transparent to the VM asit would simply reach out to access the storage and the module on thesecond site would re-direct the request to the storage on the secondsite. In some embodiments, the module on the second site would directthe request to the data on the second site. In some embodiments, thestorage may be kept in sync using a mirror, the VM may access a currentversion of the data, regardless of on which site the VM is located. Themodules at the first and second site may be in communication with eachother.

In some embodiments, disparate storage arrays at two separate locationsmay be enabled to appear as a single, shared array to application hosts,allowing for the easy migration and planned relocation of applicationservers and application data, whether physical or virtual. In otherembodiments, effective information distribution by sharing and poolingstorage resources across multiple hosts may enabled. In furtherembodiments, manage of virtual environment may be enabled totransparently share and balance resources across physical data centers,ensure instant, realtime data access for remote users, increaseprotection to reduce unplanned application outages, and transparentlyshare and balance resources within and across physical data centers.

In further embodiments, concurrent read and write access to data bymultiple hosts across two locations may be enabled. In otherembodiments, realtime data access to remote physical data centerswithout local storage may be enabled. In some embodiments, the virtualservice layer may be implemented by EMC's VPLEX or the like.

Refer to the example embodiment of a virtual service layer of FIG. 3. Inthe embodiment of FIG. 3, there are two sites 310, 350. Each site has arespective VM space or a space able to run virtual machine, 315, 355,SANs, 320, 330, 360, and 375 and storage 335, 380, respectively. The twosites also have a virtual service later 385, which presents virtualvolumes 325. The synchronization 390 of the storage 335 is provided bythe virtual service layer 385. In the embodiment of FIG. 3, the samevirtual volume may be exposed via the virtual service layer 385. Thisvolume may be kept synchronized so that any VM in VM Space 315 or VM inVM Space 355 accesses the same virtual volume with the same dataregardless of in which VM Space, 315, 355, the VM resides.

In some embodiments of the current disclosure, replication and datamobility may be enabled at difference geographic sites. In certainembodiments, this may be enabled by cache coherency functionality. In atleast some embodiments, the cache coherency may enable data to beconsistent over large distances and be able to be accessed at both geosites. In a particular embodiment, there may be two geo sites. In thisembodiment, if a read is performed on an area of the storage that doesnot belong to the local site, the read may be delayed and the read maybe performed on the remote site. In this embodiment, if a read isperformed on an area owned by the local site, then the read may beperformed on the local site.

In other embodiments, the geo sites may enforce a write order fidelitymechanism (WOFM) by periodically quiescing or stopping the storage andensure that the replicated data is consistent. In these embodiments, acheckpoint may be created at each site. This checkpoint may betransmitted to the other site. The other site may flush this checkpointin order to ensure it has the data as the other site. In theseembodiments, only consistent data may be written to the other site. Inthese embodiments, if a site crashes, then both sites are ensured tohave a point in time, where both sites have the same data.

A discussion of some types of virtual storage may be found in U.S. Pat.No. 7,206,863, entitled “SYSTEM AND METHOD FOR MANAGING STORAGE NETWORKSAND PROVIDING VIRTUALIZATION OF RESOURCES IN SUCH A NETWORK” issued onApr. 17, 2007, to EMC Corp, U.S. Pat. No. 7,770,059, entitled “FAILUREPROTECTION IN AN ENVIRONMENT INCLUDING VIRTUALIZATION OF NETWORKEDSTORAGE RESOURCES” issued on Aug. 3, 2010, to EMC Corp, U.S. Pat. No.7,739,448, entitled “SYSTEM AND METHOD FOR MANAGING STORAGE NETWORKS ANDPROVIDING VIRTUALIZATION OF RESOURCES IN SUCH A NETWORK” issued on Jun.15, 2010, to EMC Corp, U.S. Pat. No. 7,739,448, entitled “SYSTEM ANDMETHOD FOR MANAGING STORAGE NETWORKS AND PROVIDING VIRTUALIZATION OFRESOURCES IN SUCH A NETWORK USING ONE OR MORE ASICS” issued on Nov. 17,2009, to EMC Corp, U.S. Pat. No. 7,620,774, entitled “SYSTEM AND METHODFOR MANAGING STORAGE NETWORKS AND PROVIDING VIRTUALIZATION OF RESOURCESIN SUCH A NETWORK USING ONE OR MORE CONTROL PATH CONTROLLERS WITH ANEMBEDDED ASIC ON EACH CONTROLLER” issued on Nov. 17, 2009, to EMC Corp,U.S. Pat. No. 7,225,317, entitled “SYSTEM AND METHOD FOR MANAGINGSTORAGE NETWORKS AND FOR MANAGING SCALABILITY OF VOLUMES IN SUCH ANETWORK” issued on May 29, 2007, to EMC Corp, U.S. Pat. No. 7,315,914,entitled “SYSTEMS AND METHODS FOR MANAGING VIRTUALIZED LOGICAL UNITSUSING VENDOR SPECIFIC STORAGE ARRAY COMMANDS” issued on Jan. 1, 2008, toEMC Corp, and U.S. Pat. No. 7,216,264, entitled “SYSTEM AND METHOD FORMANAGING STORAGE NETWORKS AND FOR HANDLING ERRORS IN SUCH A NETWORK”issued on May 8, 2007, to EMC Corp, all of which are hereby incorporatedby reference. A discussion of mirroring may be found in U.S. Pat. No.7,346,805, entitled “PROTECTION OF MIRRORED DATA” issued on Mar. 18,2008 to EMC Corp, which is hereby incorporated by reference.

Journal Based Replication with Point in Time Access

In most embodiments, each site of a geographically separatedactive/active storage cluster of sites may have a write cache; the cachemay be used to maintain write order fidelity. In most embodiments, eachsite may reserve a portion of a volume, so that other sites may not beable to write to the reserved portion of the volume, unless the siterequests a reservation to this portion of the volume. In certainembodiments, the write cache may contain the changes that occur to thereserved portions of the volume.

In some embodiments, the write order fidelity mechanism may periodicallyquiesce both sites and may create a consistency check point or a changeset. In certain embodiments, the data from the caches of each site maybe asynchronously transferred to a replica site. In most embodiments,the change set data may periodically be flushed on the backend storageto enable the backend storage has a consistent point. In at least someembodiments, before a change set is flushed the system may read the undoof the change set and send it to a replication appliance (which may be aphysical or virtual appliance or run as a process inside the cluster orsites). In some embodiments, the appliance may write the data to an undojournal and the system may flush the data to the backend storage.

In some embodiments of the instant disclosure, the sites or cluster ofsites may be fractured or broken. In certain embodiments, this mayenable active production to occur on one of the sites while enabling asecond site to be rolled to a point in time (PIT). In at least someembodiments, this may enable a user to examine and use a PIT whileactive production may be occurring on another site.

Refer now to the example embodiment of FIG. 4. In the example embodimentof FIG. 4, there are two sites, site 419 and site 450. In this example,Site 410 and site 450 each have a VM space 415, 455. In this embodiment,the VM spaces 415 and 455 have reserved a portion of virtual volumes 425and 430 which part of virtual service layer 485. In this embodiment VMspace 415 has reserved portion 427 of volume 425 and VM space 455 hasreserved portion 432 of Virtual Volume 430. As noted herein, the virtualvolumes 425 and 430 appear to the VM Spaces 415 and 455 to be the samevolume. In FIG. 4, each site also has a Replication ProtectionAppliance, 420 and 475 respectively, a journal, 440 and 480respectively, and Write Cache 435 and 470 respectively.

Refer now to the example embodiments of FIG. 5 and FIG. 6. The exampleembodiments of FIGS. 5 and 6 illustrate creating any point in timejournal at each site. Periodically the storage on sites 410 and 450 isquiesced (step 500). A synchronization point i.e. a change set iscreated (step 510), and storage may be un-quiesced. The write cachedata, 435 and 470, is transferred to the other sites (step 520). In someembodiments, the write cache may be transferred asynchronously. When achange set is to be flushed to the backend storage, the undo data forthe change set is read from backend storage (at each of the sites) (step525). The undo data is written to the RPAs 420 and 475 (step 530), eachRPA 420, 475 writes the undo data to the journals 440 and 480. The datain the write caches 435 and 470, respectively, is written to the volumes(step 540). Once the change set is flushed the RPA is indicated a changeset flush is completed and the DPA create a snapshot i.e. a consistentpoint in time that the user will be allowed to access (step 550).

Refer now to the example embodiments of FIGS. 6 and 7, which illustratea user examining previous point in time (PIT). In certain embodiments,the user may seek to restore a corrupted file. In other embodiments, theuser may seek to restore the whole system to a previous PIT. In furtherembodiments, the user may want to perform tests on a previous PIT. Theuser request access to a specific point in time (705),

The cluster or sites 610 and 650 are split (step 710). The IOs arestopped on the non-active site, in this embodiment site 650 (Step 720).The IOs in the write caches 635 and 670 are flushed to the volumes 625and 630 (Step 730). The non-active site, 650, is rolled to point in time(PIT) 632 (Step 740). In certain embodiments, when rolling the data fromthe undo log is read, the redo for the undo data may be read from thevolume, may be written to a redo log, and the undo data may be appliedto the volume. PIT 632 is exposed to a user (Step 750). In mostembodiments, the user may now test the data at the non active site andIOs may continue to flow to the active site. In most embodiments, thestorage is an active/active storage where both sites may expose thelogical unit with the same identity, when the data is exposed to theuser in a test mode, the system may expose the logical unit of thedifferent point in time with a different SCSI identity.

Refer now to the example embodiments of FIGS. 6 and 8. The user isenabled to access the PIT 632 (step 810) and is enabled to read andwrite from the image. Changes to the PIT 632 are written to the UNDOstream in journal 680 (step 820). A decision is made if it is desirableto revert to the PIT 632 of volume 630 to the active site or to continuewith the current image of volume 625 on active site 610, i.e. discardthe changes made to PIT 632 and make the two (or more) sitesactive/active again. If the decision is made to keep the current imageof volume 625 on active site 610, changes made while accessing PIT 632are discarded (step 835) by rolling the undo stream to original data ofpoint in time 632. In most embodiments, during this roll no redo datamay be written because it is desired to discard the changes made. Inmost embodiments, the image of volume 630 is rolled to the latest PIT injournal 680 (step 845) by reading the data in the redo log, reading theundo of the redo data from volume 630, writing the undo data to the undostream and writing the redo data to volume 630. The sites 610, 650 aresynchronized (step 850). In certain embodiments, the synchronization maybe performed by the virtualization layer, which may track the changesthat happened in the active site, and the virtualization may discard thechanges which happened on site 650 since the changes where reverted. Ifa decision is made to access the selected point 632 in time access tothe active site 610 is stopped (step 855). The redo log is discarded(step 857). The hosts, in this embodiment, are virtual machines in VMspace 615, are brought down (i.e. stopped from accessing the volumes)(step 860). Both sites 610 and 650 are synchronized using the changestracked by the virtualization layer (step 865). The hosts in VM spaces615, 655, are brought up (step 870).

In most embodiments, when the sites are being synchronized undo for allIO operations to the virtualization layer may be written to the undologs. In at least some embodiments, new snapshots may not be created onthe site which the data is being updated. In certain embodiment, if thedata of PIT 632 was to be kept, until synchronization of this PITfinished, new snapshots may not be created in site 610. In otherembodiments, if the data of site 610 was kept, no new snapshots may becreated at site 650 until initialization ends. In most embodiments,there may be no new snapshots is that until the sites are synched, thedata may not be in a consistent state.

The methods and apparatus of this invention may take the form, at leastpartially, of program code (i.e., instructions) embodied in tangiblemedia, such as floppy diskettes, CD-ROMs, hard drives, random access orread only-memory, or any other machine-readable storage medium. When theprogram code is loaded into and executed by a machine, such as thecomputer of FIG. 9, the machine becomes an apparatus for practicing theinvention. When implemented on one or more general-purpose processors,the program code combines with such a processor 903 to provide a uniqueapparatus that operates analogously to specific logic circuits. As sucha general purpose digital machine can be transformed into a specialpurpose digital machine. FIG. 10 shows Program Logic 1034 embodied on acomputer-readable medium 1030 as shown, and wherein the Logic is encodedin computer-executable code configured for carrying out the reservationservice process of this invention and thereby forming a Computer ProgramProduct 1000. The logic 1034 may be the same logic 940 on memory 904loaded on processor 903. The program logic may also be embodied insoftware modules, as modules, or as hardware modules.

The logic for carrying out the method may be embodied as part of thesystem described below, which is useful for carrying out a methoddescribed with reference to embodiments shown in, for example, FIG. 5and FIG. 7. For purposes of illustrating the present invention, theinvention is described as embodied in a specific configuration and usingspecial logical arrangements, but one skilled in the art will appreciatethat the device is not limited to the specific configuration but ratheronly by the claims included with this specification.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, the present implementations are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

What is claimed is:
 1. A computer program product for use in replicationcomprising: a non-transitory computer readable medium encoded withcomputer executable program code for replication of data, the codeconfigured to enable the execution of: quiescing IO commands to sites ofan active/active storage system, the active/active storage system havingat least two storage sites communicatively coupled via a virtualizationlayer, wherein the virtualization layer enables cache coherency betweeneach of the at least two storage sites, wherein the virtualization layerenables simultaneous, active/active, concurrent read/write access to asame virtual volume at both of the sites; creating a change set for thesame virtual volume; unquiescing IO commands to the sites; transferringdata of the change set to the other sites of the active/active storagesystem by the virtualization layer; and flushing the data to the sites.2. The code of claim 1 wherein the flushing comprises: reading, by thevirtualization layer, an undo of the data to be flushed in the changeset; writing the undo data corresponding to the data in the write cacheto a data protection appliance (DPA); writing, by the DPA, the undo datato a journal; and writing the data to the backend storage system by thevirtualization layer.
 3. The code of claim 2 where the DPA is a virtualmachine; and the code further enables: writing the data from the writecache to a volume; notifying the DPA a full change set has been flushed;and creating a bookmark at the undo journal by the DPA.
 4. A computerprogram product for use in replication enabling: a non-transitorycomputer readable medium encoded with computer executable program codefor replication of data, the code configured to enable the execution of:fracturing a cluster of an active/active storage system, wherein thecluster includes at least two sites, wherein an active/active storagesystem that is not fractured includes a virtualization layer, whereinthe virtualization layer enables simultaneous, active/active, concurrentread/write access to the same virtual volume at both of the sites,wherein the virtualization layer enables cache coherency between each ofthe at least two storage sites; stopping TO on a first site of thecluster; and rolling to a point in time on a first site of the clusterthat is consistent with at least a second site of the cluster before theat least two sites were fractured.
 5. The code of claim 4 furtherenabling: flushing a write cache on a second site of the cluster to thefirst site of the cluster.
 6. The code of claim 4 further enabling:requesting, by a user, access to a point in time; writing undoinformation generated by rolling to the point in time to a redo log;exposing the point in time to the user; enabling the user to access thepoint in time data; and storing changes to the point in time in an undostream.
 7. The code of claim 6 further enabling: exposing the point intime to the user, where in the logical unit identity is different thanthe original logical unit identity.
 8. The code of claim 7 furtherenabling: shutting down hosts on second site, before stopping access,and restarting hosts when access restored.
 9. The code of claim 4further enabling: determining to revert to the point in time; and basedon a positive determination stopping access to the second site,discarding the redo log on the first site, making the sites notfractured, synchronizing the sites to the point in time and allowingaccess to the second site.
 10. The code of claim 9 further enabling:based on a negative determination, stopping access to the first sitediscarding the changes made at point in time; rolling the first site tothe latest point in time using the redo log, making the sites notfractured and synchronizing both sites to an image on the second site;and restoring access to the first site.
 11. A computer implementedmethod use in replication, comprising: quiescing IO commands to sites ofan active/active storage system, the active/active storage system havingat least two storage sites communicatively coupled via a virtualizationlayer, wherein the virtualization layer enables cache coherency betweeneach of the at least two storage sites, wherein the virtualization layerenables simultaneous, active/active, concurrent read/write access to asame virtual volume at both of the sites; creating a change set for thesame virtual volume; unquiescing IO commands to the sites; transferringdata of the change set to the other sites of the active/active storagesystem by the virtualization layer; and flushing the data to the sites.12. The method of claim 11 further comprising: reading, by thevirtualization layer, the undo of the data to be flushed; writing undodata corresponding to the data in the write cache to a data protectionappliance (DPA); writing, by the DPA, the undo data to a journal; andwriting the data to the backend storage system by the virtualizationlayer.
 13. The method of claim 11 where the DPA is virtual machine andthe method further comprising: writing the data from the write cache toa volume; notifying the DPA a full change set has been flushed; andcreating a bookmark at the undo journal by the DPA.
 14. A computerimplemented method comprising: fracturing a cluster of an active/activestorage system, wherein the cluster includes at least two sites, whereinan active/active storage system that is not fractured includes avirtualization layer, wherein the virtualization layer enablessimultaneous, active/active, concurrent read/write access to the samevirtual volume at both of the sites, wherein the virtualization layerenables cache coherency between each of the at least two storage sites;stopping IO on a first site of the cluster; and rolling to a point intime on a first site of the cluster that is consistent with at least asecond site of the cluster before the at least two sites were fractured.15. The computer implemented method of claim 14 further comprising:flushing a write cache on a second site of the cluster to the first siteof the cluster.
 16. The computer implemented method of claim 14 furthercomprising: requesting, by a user, access to a point in time; writingundo information generated by rolling to the point in time to a redolog; exposing the point in time to the user; enabling the user to accessthe point in time data; and storing changes to the point in time in anundo stream.
 17. The computer implemented method of claim 16 furthercomprising: exposing the point in time to the user, where in the logicalunit identity is different than the original logical unit identity. 18.The computer implemented method of claim 14 further comprising:determining to revert to the point in time; and based on a positivedetermination stopping access to the second site, discarding the redolog on the first site, un-fracturing the sites by unfracturing thevirtualization layer, synchronizing sites to the point in time andallowing access to the second site.
 19. The computer implemented methodof claim 18 further comprising: based on a negative determination,stopping access to the first site discarding changes made at the pointin time; rolling the first site to the latest point in time using theredo log, making the sites not fractured by connecting thevirtualization layer and synchronizing both sites to an image on thesecond site; and restoring access to the first site.
 20. The computerimplemented method of claim 14 further comprising: shutting down hostson second site, before stopping access, and restarting hosts when accessrestored.